1. Technical Field
U.S. Pat. No. 5,576,056 discloses a method of inhibiting coal oxidation in a coal pile comprising coating all the surfaces of coal exposed to air with an oxidation inhibiting amount of a composition consisting essentially of a water soluble cationic polymer diluted in an aqueous solution.
2. Background
Coal exportation has been a growing market but the self-heating property of coal has presented many risks. Upon exposure to air, coal will oxidize and generate heat. The heat that is generated accumulates and can cause ignition of the coal body. If a mass of coal burns, then there are health risks involved for the workers that will be exposed to the fumes, environmental concerns as large amounts of noxious gases are emitted, and the product is lost in transit. Coal can spend up to a couple weeks in a railcar and up to a month in the hold of a ship. A large mass of coal given that much time to heat up is going to be prone to spontaneous combustion. Due to the risks involved with spontaneous combustion, a treatment method to inhibit this reaction is needed.
Spontaneous combustion of coal is the process of self-heating resulting eventually in its ignition without the application of external heat. Coal when exposed to air absorbs oxygen at the uncovered surface. Some fraction of the exposed coal substance absorbs oxygen at a faster rate than others and the oxidation results in the formation of gases. Mainly CO, CO2, water vapor along with the evolution of heat during the chemical reaction. If the rate of dissipation of heat is slow with respect to the evolution of heat by oxidation there is a gradual buildup of heat and temperature reaches the ignition point of coal thereby causing fire.
Favorable conditions for spontaneous heating are accumulation of heat caused by a rise in temperature and hence an increase in the reaction rate. Although, at ambient temperature, the reaction can be so slow that it is unnoticed, when heat accumulates the temperature is raised and, the reaction rate increases exponential. The increased rate of reaction can be described by Arrhenius law, ν=crcoAe(Ea/RT), where ν=reaction (mol/g·s), cr=combustible concentration (kg/m3), co=oxygen concentration, A=Arrhenius Frequency Factor (s−1 or s−1C1-n), Ea=Activation energy (kJ/mole), R=universal gas constant=8.314 J/mole·K, and T=temperature (K). The oxidation rate equation of coal was established according to the chemical kinetic equations of spontaneous combustion of coal. Coal oxidation is an exothermic reaction. The equation of this exothermic reaction is: coal+O2→production+Q, where Q is the oxidation reaction heat, J/mol.
The law of mass action in chemical kinetic reactions shows that the reaction rate is a function of the concentration of the reactant at a given temperature. The rate of reaction of spontaneous combustion of coal is as follows: K′=kCmcCnO2 where K′ is the rate of reaction, k the reaction rate constant and m+n the reaction index. Mass experiments have shown that temperature has a great effect on the rate of chemical reaction. Under normal conditions, as the temperature rises 10° C., the reaction rate will increase approximately 2 to 4 times. The rate of coal oxidation increases quickly as the reaction temperature rises. The rate equation of chemical reaction is: k=k0(−E/RT) where k is the reaction rate constant. Various units can be used according to different circumstances. For example, the amount of oxide production per unit time is expressed in mol/s; k0 is the frequency factor, with the same unit as k; E is the activation energy, J/mol, and R the gas constant: R=8.314 J/mole·K. Using the mathematical model of the shortest spontaneous coal combustion period and the basic theory of thermodynamics, the equation for the time of spontaneous combustion of coal at the prevailing temperatures is as follows: t=(Cp(Tkp−T0)+Wpλ/100)/(3600×24KcpCO2Q), where t represents the time from normal temperature to the critical temperature, d; T0 is the original temperature of the coal-rock mass; Tkp is the critical temperature causing the coal temperature to rise, K; Wp is the total content of water in the coal, %; Cp is the average specific heat of the coal from normal temperature to the critical temperature, J/kg·K); λ represents the absorption heat when water evaporates J/kg; Q is the absorption heat of coal absorbing oxygen, J/m3 and Kcp is the velocity constant of absorption of oxygen during the period of (T0−Tkp, m3/(kg·s).
The propensity of colliery wastes to combust spontaneously, is related to the specific ability of seams or splits of seams to self-heat during or after mining. The instances of burning coal wastes are increasing with the increase in the percentage of coal mined by open cut methods. Wastes created in open cut mining often contain coal from seams and splits that for either reasons of quality and/or thickness are not reclaimed. This coal is often blended with the overburden by the heavy machinery used in mining, and if liable to spontaneous combustion results in numerous pockets of heating across and through the wastes. This process of spreading the source of heating through the overburden makes reclamation of mined out areas very awkward and in two cases reclamation has failed over large areas due to spontaneous combustion.
Spontaneous combustion in washery rejects has also been a problem in with coal from certain seams. Washery rejects can be seen burning after many years in a number of locations in New South Wales. The extent of environmental impact of such reject fires however is less in potential than that from burning overburden, in that the rejects are normally more concentrated and not as extensive (and therefore more easily disposed of by deep burial) as overburden. Colliery rejects are also often able to be re-washed to obtain otherwise lost coal values, while at the same time reducing the propensity for spontaneous combustion.
Other sources of environmental damage from coal spontaneous combustion are burning coal stockpiles and in situ coal seams. These sources of pollution are normally short lived due to the economic cost of losing mined or minable coal.
There are number factor which contribute to the process of spontaneous combustion of coal. The most important parameters involved in the process of spontaneous combustion of coal are: Factors inherent to coal—size of the coal particles and surface area, moisture content, coal composition, quality and rank of coal, and heat conductivity of the particles; Extrinsic conditions—degree of compaction, temperature, barometric pressure, oxygen concentration, and dimensions and shape of stockpile.